a) Field of the Invention
The present invention relates to a method for confirming whether the diameter of a selected or designated drill in a PWB (printed wiring board) working machine has a desired diameter or not.
b) Description of the Related Art With the increasing packaging density of electronic devices mounted on printed wiring boards in recent years, drills for boring such printed wiring boards are becoming smaller in diameter, leading to a substantial increase in the types of drills. On the other hand, PWB working machines are in many instances operated over a considerable duration. It is therefore now increasingly recognized that many drills of the same diameter should be stored in each PWB working machine.
In general, drills are manually placed in their corresponding storage positions specified in accordance with their diameters in a storage of a PWB working machine. During machining, the PWB working machine automatically replaces the drills and performs the machining. Namely, upon production of a drill replacement command, a drill which has been used until that time is automatically returned to the position where the drill was stored, in other words, to its home position and a drill conforming with the command is then automatically taken out from another storage position. This drill replacement is executed correctly without making any failure in the selection of these storage positions. However, due to the above-described increases in both the type and number of drills, there is increased risk that a drill may be manually placed in a wrong storage position, namely, a drill may be located in a storage position allocated to a drill of a different diameter. Such error has heretofore been found by inspecting each printed wiring circuit after completion of all bores drilled. This conventional method however involves the problems that the above inspection requires appreciable labor and time and a printed wiring board has to be thrown away where it has been drilled with a diameter other than that desired.
With a view toward overcoming such problems, drill diameter detectors have recently been developed to detect the diameter of each drill as it is brought into use on a PWB working machine. One example of such drill diameter detectors will hereinafter be described with reference to FIGS. 1 and 2 of the accompanying drawings.
FIG. 1 is a simplified plan view of a typical drill diameter detector, in which there are shown a light source 1, lenses 2,3 and a photodetector array 4. These elements make up the drill diameter detector which is identified generally at numeral 5. Designated at numeral 6 is a drill. The drill 6 has the cross-sectional shape shown in detail in FIG. 2. The diameter of the drill 6 is defined by a maximum dimension D.
To detect the diameter of the drill 6, the drill 6 which has been taken out automatically from a specific storage position in a storage and is a target of the detection is placed at the position, which is indicated in FIG. 1, in a direction perpendicular to the drawing sheet while the light source 1 is lit. Light from the light source 1 is converted to parallel rays by the lens 2, focused through the lens 3, and then projected against the photodetector array 4. Here, because of the existence of the drill 6, a shadow is formed as indicated by hatching. To ensure that the shadow reflects the maximum dimension D, the drill 6 is rotated at a low speed and the maximum number of photodetectors shaded in the photodetector array 4 is then counted, whereby the maximum dimension D, namely, the diameter of the drill, can be detected.
Detection of the diameter of the drill 6 with the drill detector 5 before working makes it possible to drill a bore of precise diameter in a printed wiring board. The drill 6 is repeatedly interchanged and used many times until a maximum number of bores preset for the drill 6 is reached. The diameter of the drill 6 is checked prior to each replacement by the drill 6.
Incidentally, chips of copper foil, resin and the like tend to stick on the drill 6 in the boring operation of a printed wiring board. When the use of the drill 6 is finished and the drill 6 is replaced by another drill, the drill 6 is stored, with the chips of copper foil, resin and the like still adhering to it to its specified position in the storage. When the diameter of the drill 6 is checked by the drill diameter detector 5 upon next use of the drill 6, an inaccurate measurement results due to the chips of copper foil, resin and the like stuck on the drill 6. This results in the selection of the drill 6 being canceled despite it being of the desired diameter.